The file containing the types may be corrupt for some reason. Have a look at the Manage file types and download actions in Firefox support article and remove the file called handlers.json in your profile folder.

Additionally, make sure there is no other software interfering with Firefox settings - the file could be reverted by some application.

Thanks. I've deleted the file and I'll see in the next few months if it works.

A little over two weeks and the MIME setting for PDF and Portable Document Format reverted to "Save File." Suggestions?

Screen shot of the error? It sounds more like a corrupted FF software and it can't see the right save option.

WestEnd said

Screen shot of the error? It sounds more like a corrupted FF software and it can't see the right save option.

There's no "error," just randomly changes the MIME to "Save File" and saves to download location instead of opening in-window.

The top option for PDF is the option to "save file", the Portable Document Format is the option to use Acrobat. I changed the top to display the options. Both should be use Acrobat. Randomly both will revert to "save file."

One possibility is to have a look at the time stamp for the handlers.json file and try to figure out or recall what happened then. The file should only be changed when modifying actions manually or at first instance for an unknown downloadable file type in Firefox (itself!), i.e. it could be 5 months old when nothing changed after setting it for the first time.

Is the time stamp more recent than the day you removed it and accepting a file type for the last time?
Do you still have a backup of the handlers.json file removed earlier, and can you put that back when Firefox is closed in order to verify this is the culprit (and some other application could have restored the file)?

Do you have any Adobe or other distributor’s software/plugin for handling PDF files installed that could restore this behavior? What does about:plugins<enter> say?

Firefox opens PDF files send as application/pdf in a tab and use the builtin PDF Viewer. If the server sends another MIME type or use content-disposition then Firefox will show the download dialog (Open with). Each automatic file action with a valid specific MIME type is stored in handlers.json. You can check the file in a text editor or drag to file in a Firefox tab to use the builtin JSON Viewer.

• https://developer.mozilla.org/en/Tools/JSON_viewer

So I've been observing this behavior and it seems to be something changing the settings, whether it is the way a website handles Acrobat files or what have you. Doesn't seem to be an issue with Firefox per se, though it would be less annoying if something wasn't adding four or more PDF mime types and I have to change it to "use Acrobat" every time something adds a new PDF mime type.